Tuesday, August 09, 2005

“If climate were the major factor driving the extinction of ground sloths, you would expect the extinctions to occur at about the same time on both the islands and the continent since climate change is a global event,” Steadman said.

Gary Haynes, anthropology professor at the University of Nevada, Reno, said Steadman’s study “clearly shows that ground sloth extinctions in the New World didn’t happen after serious changes in climate or vegetation – and that the first appearance of humans must have been the decisive factor.”

The fossil record shows the people who arrived in North America were making sophisticated tools out of stone, bone and ivory, Steadman said. These “big-game hunters” had a traumatic effect on the animals living there, he said.

More than three-fourths of the large species of mammals that roamed the North American landscape became extinct within a few thousand years, which, besides ground sloths, included mammoths, mastodons, saber-toothed tigers and giant bears, Steadman said.

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While the largest of the prehistoric ground sloths grew to the size of a modern elephant and fed on bushes and the leaves of lower branches of trees, today’s only surviving descendants are several small tree sloths whose range extends from southern Mexico to southern Brazil, he said.

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The only reason the living species of sloths survive is that they live high up in trees, where their green-algae-colored fur camouflages them, Steadman said. “God save the sloth that comes down to the ground because usually somebody is there to kill it,” he said.

Interfacing human evolution and the evolution of the sloths. An evolutionary hypothesis tested.